The invention relates to a clamping device for clamping an object, preferably plate material, to a clamping portion of said object.
Such clamping devices are in particular intended for use with loading and unloading devices, such as cranes and lifting gear for lifting plate-like material, such as metal plates, elongate objects, etc. With the clamping device, it is also possible to lift or displace objects having a part which is suitable to be clamped.
Such a clamping device is known, for example, from U.S. Pat. No. 4,641,877 and NL-A-7414215. These documents describe a clamping device which is provided with a first clamping part and a second clamping part. The first clamping part is rotatably connected to a frame and connected to a movable lifting eye by means of an arm. If, for example, a panel is positioned between the clamping parts, the lifting eye will slide in the frame when the clamping device is lifted, as a result of which the arm will cause the first clamping part to rotate. As a result thereof, teeth of the first clamping part will contact the plate, will lock onto the plate, and in addition the first clamping part will exert a force on the plate in the direction of the second clamping part so that the plate is clamped between the first and second clamping part. In order to achieve a sufficiently strong clamping action, the object to be lifted has to have a minimum weight. In particular with thin and lightweight plate material, it has been found that the clamping action is in many cases insufficient. Consequently, in addition to a maximum acceptable weight (also referred to as the work load limit or WLL), a minimum acceptable load will be specified for a clamping device. Depending on the hardness of the material to be clamped, this may be 5-10% of the WLL.
GB-A-1,385,772 of applicant discloses a hoist clamp for clampingly gripping plates. That hoist clamp comprises a toggle lever system, pivotally coupled. A first lever of this system being pivotally connected to a hoisting eye by means of a pin movable in a slot formed in the side of the clamp. A second lever is coupled to an axis which has a fixed position with respect to the side of the clamp and has a working surface which is eccentric with respect to the axis. The clamp further has a pressure surface on a wedge which has a toothing on its slanted side. Fixedly coupled to the side of the clamp is a member, also having a toothing, and on the member the wedge is slidingly mounted with the toothed sides directed towards each other. The wedge is spring-biased downwardly. The toothing is such that when a pressure force is applied, the toothing blocks upward movement of the wedge on the member. A further lever can block the wedge in its upward position.
When a plate is positioned between the working surface and the pressure surface, the wedge is released using the further lever, and the wedge is pressed downward, clamping the plate. Another lever, blocking the toggle lever system, is now released. When starting to hoist, the toggle levers are pulled, transferring the hoisting force to the working surface, this strongly clamping the plate between the working surface and pressure surface.
A disadvantage of this construction is that this clamp does not allow relatively light objects to be hoisted securely: The clamping force during hoisting always depends on the weight of the object.